This invention relates to a battery charger, and more particularly to a battery charger most suitable for charging lithium ion rechargeable batteries.
Various batteries such as lithium ion rechargeable batteries, the nickel cadmium batteries, nickel hydrogen batteries, etc. are representative of rechargeable batteries capable of repeated discharge and recharge. The lithium ion rechargeable battery is being recognized as a high capacity rechargeable battery. To optimally charge these rechargeable batteries, charging methods appropriate to the battery's characteristics have been adopted. For example, a charging method for the lithium ion rechargeable battery is described in Japanese Patent Disclosure 2-192670 (1990). In this method, the battery is charged with a constant voltage and charging is stopped when the charging current drops below a prescribed value. Otherwise in this method, the battery is first charged with a constant current until a prescribed voltage is reached then it is charged with a constant voltage and charging is stopped ceased when the charging current drops below a prescribed value. Further, for nickel cadmium and nickel hydrogen batteries, charging with a constant current is carried out until a prescribed battery voltage is reached, at which point full charge is assumed and charging is stopped.
If a battery is continually charged without restraint, battery voltage will rise abnormally high causing battery performance degradation. FIGS. 1A-1C show graphs of battery voltage and current characteristics as charging proceeds. In this figure, the characteristics of a degraded battery or a battery when ambient temperature is low are shown by the broken lines A. In other words, the lines A show characteristics for a battery with a reduced actual battery capacity. The reason that a long time is required to charge a degraded battery or one with a low ambient temperature is because a internal resistance of the battery becomes large. As shown in this when a battery is charged with a constant voltage, the charging current gradually decreases as a full charge is approached. The current approaches zero but does not become exactly zero. This small non-zero charging current is consumed in secondary reactions such as electrolyte dissociation. For this reason, when a battery is charged for a long period, for example a by constant voltage charging, the battery can be over-charged, thereby degrading its performance.
A method to limit charging time with a protection timer has been adopted to prevent over-charging. In this battery charging method, when a specified time has elasped since the start of charging, even if the full battery charge has not been detected, the charging is forced to stop by the protection timer. As reported in Japanese Patent Disclosure 4-88836 (1992), the protection timer can be set to a suitable time depending on the battery to be charged (for example, a battery of different capacity).
Further, constant current charging time can be limited by a protection timer for a charging method that stops charging at a full charge determined when the charging current drops below a prescribed level during constant voltage charging performed after the battery voltage rises above a prescribed level during the constant current charging. The constant current charging time is set such that constant current charging cannot be performed over a long period of time. When the protection timer time has elapsed, the battery charger forces the charging to cease.
As described above, the protection time of the protection timer can be appropriately set depending on the type of battery. However, this alone does not sufficiently protect the battery. This is because even for the same type of battery, the charging characteristics vary depending on the battery's remaining capacity.
FIGS. 2A-2B show the battery voltage V and charging current I versus charging time characteristics a lithium ion rechargeable battery with constant current or constant voltage charging. FIG. 2A is for the case of little remaining battery capacity and FIG. 2B is for the case of considerable remaining battery capacity.
As shown in FIG. 2A, the protection timer is set to a time TS that is capable of protecting the battery after charging it for a time just long enough to obtain a full charge from a state of a low capacity (namely, for a time TO where the charging current goes close to zero). A battery as shown in FIG. 2B with considerable remaining battery capacity charges to full charge in a short time. Since the time from TO in FIG. 2B, where the battery reaches full charge, to the time TS, where the protection timer times out, is extremely long, it is difficult to say that the protection timer's performance is sufficient.